Spec compatibility for iteratorClose condition. (#6094)

* for-of: IteratorClose spec compatibility.

See #3:
https://tc39.github.io/ecma262/#sec-iteratorclose

* Update spec fixtures for for-of.

* Fix IteratorClose case for remap-async-to-generator.

* Fix IteratorClose case for async-generator-function test output.

* Modify few tests according to iteratorClose fix.

* Fix iteratorClose for helpers.slicedToArray also.

* Update iteratorClose fixture for commonjs.

packages/babel-helpers/src/helpers.js

@@ -542,7 +542,7 @@ helpers.slicedToArray = template(`
         _e = err;
       } finally {
         try {
-          if (!_n && _i["return"]) _i["return"]();
+          if (!_n && _i["return"] != null) _i["return"]();
         } finally {
           if (_d) throw _e;
         }